The invention concerns a radiation unit for a fixation device to fix toner material on a printed stock surface for an electrophotographic printer according to a method for exposure and fixation of toner material on a printing stock surface.
In electrostatic printing a latent image is produced on the surface of a cylinder (photoconductor drum) coated with a photoconductor material. Toner material applied by means of a development station adheres to the electrostatically charged areas of the photoconductor drum, which represent the latent image. The developed latent image is transferred in a subsequent step to a printed stock surface guided along the photoconductor drum. Another variant transfers the developed latent image only to an intermediate carrier and from this to the printed stock surface. Because of this, the developed latent image is made visible and imaged on the printed stock surface.
The application and fixation process, as well as the cooling times of the toner material, are problems, among other things, which must be considered, in order to avoid wiping off of image parts and undesired lengthening or disturbance of the printing process. Different solution proposals to prepare fixation devices for fixation of toner material on the printed stock surface have therefore been offered. Methods were developed that avoid the drawbacks of fixation by contact with the printed stock surface by means of radiation.
U.S. Pat. No. 5,526,108 describes an xe2x80x9celectrostatographicxe2x80x9d printer with an image station to image a latent electrostatic image on the surface of the cylinder, a toner developer station for development of the latent image, in order to produce a toner image, and a toner transfer station to transfer the toner image to a moving surface. The invention also includes a fixation station to fix the toner image on the printing stock surface, consisting of two pairs of radiating heat sources, in which the wavelength with the maximum energy delivery lies in the infrared spectral range. The temperature of the heat sources lies in the range from 150xc2x0 C. to 300xc2x0 C.
European Patent Application No. EP 0 992 864 discloses fixation of an ink on a sheet-like and/or endless support, especially toner powder on copier paper and/or laser printing paper, in which the ink is heated, in order to achieve a permanent bonding with the support, and especially cross-linking of the toner. In this case, the ink is exposed to infrared radiation, especially an infrared lamp at emission temperatures of 2500K or higher, so that the ink is heated by absorption of at least part of the infrared radiation and fixed. EP 0 992 864 discloses no ink-independent fixation and the disclosed ink fixation is therefore usable for color printing only with additional absorber materials in the toner material, during whose application shortcomings occur in color space and triboelectric behavior (see page 6, lines 14-28). Another shortcoming in both of the aforementioned methods are the low radiation powers and the resulting large dimensions of the radiation units.
When radiation is used on a printing stock surface, the problem of radiation homogeneity exists, among other things, i.e., the printing stock surface is exposed unequally. Because of this, energy losses occur, since fractions of the toner material are sometimes exposed more than necessary for fixation, while other fractions of the toner material are still not sufficiently fixed. Because of the increased energy effect on fractions of the printing stock, the structure and color of the more strongly exposed printing stock surface can also be altered up to printing stock curling, lifting of the printing stock surface from the printing stock and deviations of the desired color fraction and spot formation of the printing stock.
A task of the invention is to offer a compact radiation unit for a fixation device for fixation of colored and black toner material on a printing stock surface in a printer. Another task of the invention is to avoid structural changes, color changes, printing stock curling, lifting of the printing stock surface from the printing stock, deviations of the desired color fraction and spot formation on the printing stock. Another task of the invention is to offer a radiation unit that fixes toner material on the printing stock independently of color. According to the invention, a radiation unit for a fixation device for fixation of toner powder for an electrophotographic printer is proposed, in which the radiation wavelength lies essentially in the ultraviolet range and includes at least one reflector. By high power densities in the ultraviolet spectral range and appropriate reflection, a radiation unit is offered for the first time with high power and relatively small dimensions. By using quartz glass as bulb material of the radiator of the radiation unit, the radiation in the ultraviolet C range is only slightly attenuated, since quartz glass has low absorption in the ultraviolet (UV) range, i.e., the wavelength range from 200 nm to 380 nm. As an alternative to quartz glass, other UV-transparent materials, like, sapphire, can be used as bulb material. Advantageously, the employed radiator can be a mercury radiator of high power with an operating voltage per unit length in the range from about 70 V/cm to 15 V/cm, in which the mercury radiator delivers the desired radiation wavelength cost-effectively and efficiently.
The power of the radiator (10, 11, 100) is adjustable in the range from 20% to 100% of its nominal power and the radiator can be operated for a period of less than 10 seconds even with a power of 200%, if the power delivery averaged over a longer period does not exceed 100%.
The reflector connected to the radiation unit is either symmetrical or asymmetrical. The asymmetric reflector is particularly advantageous for fixation of specific toner materials.
A grid or screen-like diaphragm can be arranged in front of the radiator, which homogenizes the non-homogeneous intensity profile of the emitted light produced by the radiation unit in the direction of the radiator bulb and therefore adjusts it to the application purpose. Light reflection can be present in the middle of the diaphragm that is stronger in comparison with the ends.
According to the invention, radiation to fix the toner material occurs essentially indirectly, i.e., the light emitted by the radiation unit is reflected at least once, so that high uniformity and homogeneity of the radiation and avoidance of adverse changes on the printing stock are achieved.
The radiation unit can have at least two radiators, each with at least one reflector, arranged on opposite sides, between which a conveyor belt with the printing stock is arranged. The toner material is therefore advantageously exposed uniformly by two radiators, each with one reflector, in which reflection of the radiation can be diffuse and/or directed. Because of this, two-sided essentially reflected exposure of the printing stock is then possible.
An additional reflector can also be mounted in the radiation unit. By means of this variant, the fire hazard in the radiation unit is also reduced, since this arrangement of the radiator and reflectors has a more limited risk, in comparison with the prior art, that, in the case of a paper jam in the printer, for example, the printing stock will come in contact with the hot radiators and hot reflectors.
An additional reflector is arranged above the conveyor belt to increase energy utilization, which is flat and covers the space above the conveyor belt, so that a higher percentage of the radiation falling on the additional reflector is reflected in the direction of the printing stock.
The reflectors and/or the conveyor belt contain a heat-resistant material to guarantee their longer lifetime, to avoid radiation damage from the high-energy radiation and a significant reduction in fire hazard. The reflectors contain or consist of Teflon(copyright), barium sulfate and/or aluminum. The conveyor belt contains or consists of Teflon(copyright). With particular advantage, a toner material is proposed having a sharp transition from its solid to its liquid or pasty state. In conjunction with the toner material, the ratio of the elastic modulus Gxe2x80x2 at the reference temperature value, calculated from the initial temperature at the beginning of the glass transition of the toner plus 50xc2x0 C., to the value of the elastic modulus Gxe2x80x2 at the initial temperature itself, can be less than 1xc3x9710xe2x88x925, preferably even less than 1xc3x9710xe2x88x927. The initial temperature of the beginning of the glass transition of the toner material is preferably determined as that temperature value at which the tangent intersects the plot of elastic modulus Gxe2x80x2 as a function of temperature before and after the glass transition. The transition from the solid to liquid or pasty state occurs within a temperature range of about 30xc2x0 K or less and between the temperature values of 70xc2x0 C. and 130xc2x0 C. The energy to be applied to the radiation unit is further reduced by this. When the described toner material is used, higher luster and high color saturation or color brilliance are also achieved on the image ultimately printed by the electrophotographic printer.